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The Solar Optical Telescope NASATM109240 The Solar Optical Telescope (NAS.A- TM- 109240) TELESCOPE (NASA) NASA The Solar Optical Telescope is shown in the Sun-pointed configuration, mounted on an Instrument Pointing System which is attached to a Spacelab Pallet riding in the Shuttle Orbiter's Cargo Bay. The Solar Optical Telescope - will study the physics of the Sun on the scale at which many of the important physical processes occur - will attain a resolution of 73km on the Sun or 0.1 arc seconds of angular resolution 1 SOT-I GENERAL CONFIGURATION ARTICULATED VIEWPOINT DOOR PRIMARY MIRROR op IPS SENSORS WAVEFRONT SENSOR - / i— VENT LIGHT TUNNEL-7' ,—FINAL I 7 I FOCUS IPS INTERFACE E-BOX SHELF GREGORIAN POD---' U (1 of 4) HEAT REJECTION MIRROR-S 1285'i Why is the Solar Optical Telescope Needed? There may be no single object in nature that mankind For exrmple, the hedii nq and expns on of the solar wind U 2 is more dependent upon than the Sun, unless it is the bathes the Earth in solar plasma is ultimately attributable to small- Earth itself. Without the Sun's radiant energy, there scale processes that occur close to the solar surface. Only by would be no life on Earth as we know it. Even our prim- observing the underlying processes on the small scale afforded ary source of energy today, fossil fuels, is available be- by the Solar Optical Telescope can we hope to gain a profound cause of solar energy millions of years ago; and when understanding of how the Sun transfers its radiant and particle mankind succeeds in taming the nuclear reaction that energy through the different atmospheric regions and ultimately converts hydrogen into helium for our future energy to our Earth. needs, it will be the reaction first discovered, and still Perhaps the best analogy for the radical new perspec- being studied, in the Sun's core that will provide that tive that the Solar Optical Telescope will give us on energy. Meanwhile, the Sun's radiation will continue to solar physics is that of the role of the microscope in drive the circulation of our terrestrial atmosphere and clarifying the nature of blood. Before the invention of will contribute to changes in our climate through subtle the microscope, there was a great deal of speculation on variations in the quantity and the spectrum of that radi- the nature of this vital fluid. Since the discovery of ation. Thus, the Sun remains, in some sense, as impor- blood circulation by the physician Harvey, we have un- tant today as it was to the ancients who worshiped it as derstood that blood somehow is involved both in "feed- a god and who sensed, even if they did not fully under- ing" the body and in removing some of its locally gener- stand, how fundamental the Sun's role is in determining ated waste products. But how? Only with observations the character and viability of our home, the Earth. of blood cells and the determination of their structure Beyond its importance to life on Earth, the Sun is also of and function could our current understanding develop. fundamental importance to stellar astronomy. The Sun is a very For this, the microscope was needed. In a like manner, typical star, and it is the only star that we can study with high the Solar Optical Telescope will provide views of the angular resolution, thanks to its proximity to us This means that solar surface that, for the first time, will permit its bas- we can study physical processes that are of fundamental sig- ic structure, "cellular" or otherwise, to be resolved. nificance in understanding stars in general, on the scale on which The Sun has long been the focus of human attention these processes are actually occurring! The history of astronomy and admiration. The arts attest nobly to this interest. In demonstrates convincingly that much of our understanding of the closing phases of Mozart's famous opera "The Mag- the physics of stars comes from understanding these physical ic Flute." the Sun-priest utters the lines (loosely trans- processes on the Sun first. It is for this reason that the Sun is lated): often called the Rosetta stone of astronomy. 'The Sun's golden rays pierce through the night, The Solar Optical Telescope will be the world's first facility that And scatter the powers of Darkness to flight.' is capable of observing the Sun on the angular resolution of a It is appropriate that we should approach the objects of typical photospheric (lower atmospheric) mean-free path for ra- nature both from the perspective of the arts as well as diation and also on the scale known to characterize major changes from that of the sciences. The Solar Optical Telescope in the gas-dynamic behavior of the atmospheric medium In gives us the opportunity of taking the next great step in simple language, these are the scales on which energy is trans- scientifically understanding the Sun, as well as those ferred in the solar atmosphere; hence, they are the scales on other fascinating astronomical objects of which it is the which the major physical processes that ultimately determine closest member, the stars. the dramatic large-scale behavior of the atmosphere take place. 6.1 I I i 4¼ 1 I 1' 1t¼ I TRANSITION 111111111 i'ii REGION OVER SUPER GRANULE - - - Mt 5.0 \lP \ c LIMB SUPER GRANULE FLOW Schematic cross section of "quiet" solar atmosphere, with supergranulatiofl flow below limb and inhomogeneous magnetic field above limb given as dashed lines. Numbers are Log 10 of the temperature. 3 Current Picture of the Sun's Atmosphere and Convection Zone The picture of the solar atmosphere and upper con- tions to the overlying atmosphere Since the convection zone vection zone that has developed over the past three dec- is not directly observable in any wavelength, except at its up- ades emphasizes the importance of magnetic fields and per boundary with the photosphere, its deeper structure can inhomogeneities. The solar atmosphere is never entirely be known only from careful studies of oscillations observed at homogeneous at any height. Material upwelling from the the solar surface (solar seismology) and from theoretical mod- deep unobservable regions of the convection zone spills eling. The only hope of understanding solar convection in de- over into the stable photosphere, where two distinct scales of tail, then, depends on using the boundary conditions obtained convective eddies are observed in the granulation and from observations at the surface and solving the differential supergranulation flow patterns. Above the photosphere. Jets equations of gas dynamics. To date, solar observations have of material called spicules shoot up from the top of the yielded a picture of convection in which elongated, hexagon- chromosphere to several thousands of kilometers into the al cells extend to some currently undertermined depth of the corona. Strong, highly localized and essentially vertical convection zone and reveal themselves in the observable magnetic fields extend from the subsurface regions through photosphere as the supergranulation flow pattern. Superim- the photosphere. These strong fields spread out and weaken posed on this larger scale flow is the smaller scale motion ob- in the higher layers, forming large-scale regions of closed fields served at the level of the photosphere and called the granula- similar to those of a dipole, as well as large regions of open tion. Theoretical estimates of flow conditions in the low pho- field lines extending into interplanetary space. Superimposed tosphere predict that this small-scale flow should be turbulent, on this complex pattern of systematic and impulsive flows and and that is largely what is actually observed there. Two other magnetic fields is a spectrum of oscillations and waves on scales of convection have been predicted theoretically, but varying scales, ranging from coherent pulsations of the entire neither has been reliably observed. These are (I) a very large- Sun with periods of up to almost 3 hours, to hydromagnetic scale. "giant cell" with a scale length extending a significant waves localized to the photosphere and chromosphere with fraction of a solar radius, and (2) a "mesoscale" convection, periods on the order of I minute. intermediate between the scales for granulation and super- Underlying this complex atmosphere is the solar convection granulation. zone, in which energy is transported by means of convective The question of the velocity structure of the convec- motions from the core where it is generated by nuclear reac- tion zone, which is intrinsically interesting, also under- lies one of the most fundamental problems in solar/stel- Let us move up higher into the solar atmosphere 4 lar astrophysics: What is the character of the dynamo again. We now know that it is filled with numerous that generates the Sun's (or a star's) magnetic field? other features whose structure and dynamics are domi- Since the motions of the subphotospheric solar plasma, nated largely by the magnetic field. This occurs be- coupled with the solar differential rotation, are ultimate- cause, above the chromosphere, the energy in the mag- ly responsible for the solar magnetic field, certain gen- netic field exceeds that stored thermodynamically in the eral constraints on the type of dynamo have already gas, reversing the picture that applied in the lower lying emerged from observations of the Sun's surface magnet- regions. Large-scale features, called prominences and ic field. For example, the dynamo must be able to ge- filaments, are well observed structures that are still on- erate the cyclic behavior in the field over a 22-year peri- ly partly understood solar phenomena for which the do- od known as the solar cycle.
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